Darth Viciate. He murdered two planets, destroyed his homeworld at the age of 10 (I believe) and was able to transfer his consciousness to other beings.
My top 3 are:
Darth Viciate - Most powerful sith ever
Tulak Hord - Greatest Duelest ever
Any one of the Dread Masters (I know that they are not powerful compared with Viciate but I love their armour)

But in reality we all know Rey will be the Mary Sue of the EU as well.

Darth Viciate. He murdered two planets, destroyed his homeworld at the age of 10 (I believe) and was able to transfer his consciousness to other beings.
My top 3 are:
Darth Viciate - Most powerful sith ever
Tulak Hord - Greatest Duelest ever
Any one of the Dread Masters (I know that they are not powerful compared with Viciate but I love their armour)

But in reality we all know Rey will be the Mary Sue of the EU as well.

Darth Viciate. He murdered two planets, destroyed his homeworld at the age of 10 (I believe) and was able to transfer his consciousness to other beings.
My top 3 are:
Darth Viciate - Most powerful sith ever
Tulak Hord - Greatest Duelest ever
Any one of the Dread Masters (I know that they are not powerful compared with Viciate but I love their armour)

But in reality we all know Rey will be the Mary Sue of the EU as well.

What about GM Luke?

I think Old Republic Jedi and Sith were stronger, so much was lost during the Old Republic wars

Darth Viciate. He murdered two planets, destroyed his homeworld at the age of 10 (I believe) and was able to transfer his consciousness to other beings.
My top 3 are:
Darth Viciate - Most powerful sith ever
Tulak Hord - Greatest Duelest ever
Any one of the Dread Masters (I know that they are not powerful compared with Viciate but I love their armour)

But in reality we all know Rey will be the Mary Sue of the EU as well.

What about GM Luke?

I think Old Republic Jedi and Sith were stronger, so much was lost during the Old Republic wars

Your sort of on point

In Knights Of The Old Republic 2 Darth Kreyla explains that the very first Sith and Jedi would make all of the Sith lords and Jedi at the time look like children playing with toy swords even Nhilius

The reason the Sith drained in power was because of their very nature. The weak would overtake the strong by force and not be smart or gifted enough to learn the secrets of the Master and so the following generations of Sith would slowly lose all the old talents, lore, and magic. It's why Palpatine is so huge because he was the first Sith in millenniums to ascend to the power level of the old Sith Masters.

The Jedi accepted almost anyone into the order regardless of power level strength. They judged by moral character rather than power like the power hungry sith so their order's power level decreased.